Formation testers are a family of devices used in the exploration for oil and natural gas deposits. In particular, formation testers obtain samples of fluids from an Earth formation surrounding a borehole. In some cases formation testers take the samples back to the surface, and in other cases the formation testers analyze the formation fluid within the formation tester and discharge the analyzed fluid into the borehole. Formation testers may be positioned proximate to the desired formation by way of a wireline after the drill string has been removed or “tripped” from the borehole, and yet other formation testers are a part of the drill string that drills the bore hole.
One of the tests a formation tester may perform down hole is a test to determine resistivity of the formation fluids (or conductivity, being the inverse of resistivity). In particular, formation fluids are drawn into the tester, and within the tester a direct current (DC) voltage is applied to an insulated button electrode in electrical contact with the fluid. The amount of electrical current that flows from the button electrode to a current return is indicative of the resistivity of the formation fluid. In some cases, the button electrode may be surrounded by one or more electrically insulated focusing electrodes, where voltage on the focusing electrodes tends to focus the electrical current flow from the button electrode.
However, the electrodes, being exposed to the formation fluid, are subject to corrosive effects of the formation fluid, and are also subject shorting by solids depositing across the electrodes and surrounding grounding surfaces. Moreover, the direct current that flows responsive to the voltage tends to cause electrolysis, which adversely affects performance of the measurements.